From 51f7e95187f127d5eadf50541943813ff57f12ba Mon Sep 17 00:00:00 2001 From: Jason Baron Date: Fri, 3 Aug 2018 17:24:53 -0400 Subject: [PATCH] af_unix: ensure POLLOUT on remote close() for connected dgram socket Applications use -ECONNREFUSED as returned from write() in order to determine that a socket should be closed. However, when using connected dgram unix sockets in a poll/write loop, a final POLLOUT event can be missed when the remote end closes. Thus, the poll is stuck forever: thread 1 (client) thread 2 (server) connect() to server write() returns -EAGAIN unix_dgram_poll() -> unix_recvq_full() is true close() ->unix_release_sock() ->wake_up_interruptible_all() unix_dgram_poll() (due to the wake_up_interruptible_all) -> unix_recvq_full() still is true ->free all skbs Now thread 1 is stuck and will not receive anymore wakeups. In this case, when thread 1 gets the -EAGAIN, it has not queued any skbs otherwise the 'free all skbs' step would in fact cause a wakeup and a POLLOUT return. So the race here is probably fairly rare because it means there are no skbs that thread 1 queued and that thread 1 schedules before the 'free all skbs' step. This issue was reported as a hang when /dev/log is closed. The fix is to signal POLLOUT if the socket is marked as SOCK_DEAD, which means a subsequent write() will get -ECONNREFUSED. Reported-by: Ian Lance Taylor Cc: David Rientjes Cc: Rainer Weikusat Cc: Eric Dumazet Signed-off-by: Jason Baron Signed-off-by: David S. Miller --- net/unix/af_unix.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c index 1772a0e32665..d1edfa3cad61 100644 --- a/net/unix/af_unix.c +++ b/net/unix/af_unix.c @@ -430,7 +430,12 @@ static int unix_dgram_peer_wake_me(struct sock *sk, struct sock *other) connected = unix_dgram_peer_wake_connect(sk, other); - if (unix_recvq_full(other)) + /* If other is SOCK_DEAD, we want to make sure we signal + * POLLOUT, such that a subsequent write() can get a + * -ECONNREFUSED. Otherwise, if we haven't queued any skbs + * to other and its full, we will hang waiting for POLLOUT. + */ + if (unix_recvq_full(other) && !sock_flag(other, SOCK_DEAD)) return 1; if (connected)